Production of amide containing cellulose



This invention is concerned with the chemical modification of cellulose.More particularly, it relates to an improved method of modifyingcellulose with such reactants as acrylamide and N-substitutedacrylamides.

In the past, various proposals have been made for modifying cellulosewith acrylamide. Therein, cellulose fibers were heated with aqueousalkali solutions and acrylamide. However, these treatments were usuallyin the presence of high alkali concentrations of some ten to fortypercent. Their purpose was to produce solutions of the modifiedcellulose, i.e., carboxyethylated cellulose, which could besubsequentlly precipitated by acid.

Subsequently, attempts were made to avoid hydrolysis of the substitutedgroup and produce carbamylethylated cellulose, rather thancarboxyethylated cellulose. These proposals required the use of loweralkali concentrations of some four to ten percent. Some useful productswere obtained. However, the operation was never successful inintroducing nitrogen contents as high as were desired. Nor were theresultant unavoidable carboxyethyl contents as low as desired.

There remained, then, a need for a process which was capable ofproducing carbamylethylated cellulose having a high carbamylethylcontent without at the same time attaining an undesirably highcarboxyethyl content. It is an advantage of the present invention thatthis has been simply and easily accomplished without introducing unusualapparatus requirements or the necessity for abnormally stringentconditions.

Moreover, previous attempts to accomplish analogous results withN-substituted acrylamides were highly unsatisfactory. Using acrylamidesit was possible to obtain maximum nitrogen contents of about 2.2 percentin the product. However, the eificiency was very poor. When anN-substituent was introduced, or as the substituent increased in size,the reaction became less effective. Using N,N'-methylenebisacrylamide,for example, the maximum nitrogen content obtainable was less than aboutone percent. When attempts were made to use such materials asN-terL-butylacrylamide, no reaction occurred. It is an advantage of thepresent invention that it is not so-lirnited.

in general, the process of the present invention may be simplydescribed. Cellulose fibers are treated with aqueous alkali solutions oflow concentration and with the desired acrylamide in the presence of asuitable Watersoluble salt present in sufficient amount. Heating atmoderately elevated temperatures of from about C. to about 130 C.,preferably about 55 to about 95 C., produces the desired result.Nitrogen contents as high as four to five or more percent are easilyobtained with acrylamide. When the required degree of substitution isattained, the treatment is stopped and the product washed and dried.

It is an advantage of the present invention that it is applicable to theproduction of the desired product Without being restricted to aparticular type of cellulose. It may be in particulate form, such asfrom Wood pulp, cotton linters, rayon flock and the like; as fibers; asyarn or thread; or as woven fabrics.

It is a further advantage of the present invention that no specialprocessing or apparatus is required. Where 35,929,232 Fatenteel Apr. 19,1962 2 the form of cellulose permits, simply stirring and heating thecellulose with the treating liquor containing the alkali, the salt andthe acrylamide in any available vessel is adequate. Where the materialis a yarn or fabric the treatment may be easily carried out inconventional equipment such as is used for dyeing operations.

It is still anotheradvantage oi the invention that the cellulose may betreated in a single operation with a single treating bath or it may beprewetted with a solution of the alkali and the salt before adding theacrylamide. It is desirable but not essential in such cases to have thesalt present during the prewetting. It need not be all added at thatstage. A portion, or if necessary all, the salt may be added as anintermediate treatment or with the acrylamide.

As noted above, the process is quite flexible as to temperaturerequirements. Reaction is of course faster at the higher temperatureswithin the range noted above. However, the caustic, or other alkali, ispresent in sufiiciently dilute solution that excessive hydrolysis doesnot occur in most cases, even over long periods.

Reaction is slower with N-substituted acrylamides than when usingacrylamide per so. However, the process is also improved and moreflexible in this respect. For example, where in the prior art noreaction was obtained in attempting to utilize such an acrylamide asN-tert-butylacrylamide, in the present process even this reactant, whichis only partially soluble in the treating liquor, may be made to reactwith the cellulose. However, in general practice the N-substituent willseldom contain more than two carbon atoms. Typical N-substitutedacrylamides which may be reacted with cellulose include N-methyL,N-ethyl, N-propyL, N-isopropyl-, N-n-butyl-, N-isobutyl-, N-t-butyl-,N-methylol, N-N methylenebis-, N-hydroxyethyl-, andN-hydroxypropylacrylamide and the like.

Accordingly, in this discussion, the term an acrylamide is usedgenerically to designate these materials and the expressioncarbamylethylated is used to designate not only amide-containingcellulose: derived from acrylamide per se but also the analogs obtainedusing these N-substituted acrylamides.

In general, the alkali used will be sodium hydroxide. However, this isnot a limitation. Any strongly reactive alkali may be used, for example,potassium hydroxide; or such strongly basic quaternary bases asbenzyltrialkylammonium hydroxide, dibenzyldimethylammonium hydroxide andthe like. It is to be understood that the term alkali solution will inmost cases be an aqueous caustic alkali solution and concentrationsherein are discussed on that basis. However, the other alkalies may besubstituted therefor in equivalent amounts.

Concentrations of aqueous alkali in the treating liquor of thisinvention will be less than four percent and usually much lower thanthose previously used. In general practice, the concentration will rangefrom about onehalf to about three parts per hundred parts of Water.Within this range, higher concentrations do produce higher reactionrates.

The weight of acrylamide used will depend to a major extent on theamount of carbamylethylation which it is desired to produce. While lesssometimes may be used, in general practice the amount will range fromabout five parts per hundred parts of water up to the solubility limit.As noted above, nitrogen contents above four percent are readilyobtainable. However, it is not necessary to produce this degree ofsubstitution to obtain the advantages of the invention. The degrees ofsubstitution previously attained are achieved more readily and theproduct is much more effectively produced. This can be measured by theratio of the carboxyethyl content to the nitrogen content. The latter isconveniently exsnaaass pressed as the weight. The former is readilyassayed as milli-equivalents per gram of product. in the productspreviously obtainable, this could not be reduced below about 1:10. Inthe process of the present invention, ratios of 1:15 to 1:20, or better,are readily obtainable.

The ratio of amount of solution to amount of cellulose is not criticalexcept in one respect. It is necessary that the treating liquor be inintimate and effective contact with the cellulose at all times.Therefore, the total 1 amount of treating liquor must be adequate tomaintain this contact. For example, to maintan an equally effectivecontact while treating yarn in a package dyeing machine, a much greatertotal volume of treating liquor is required than when stirring aparticulate form in an open kettle. That portion of the liquor inimmediate contact with the fiber should contain effective reactantamounts in terms of concentration and the minimum volume should furnishat least the required weight of reactants. However, the total volume andhence the total weight may and often will be greatly in excess of this.

Probably more important as a factor in effecting the desired result thanany other is the use of the correct amount of a suitable salt. Thepresent invention depends on the discovery that certain water-solublesalts, which are substantially unreactive with acrylarnide underreaction conditions, are capable of accelerating the reaction andincreasing the efficiency. These are salts which in aqueous solutionhave a swelling effect on cellulose. In general, the useful salts aresubstantially neutral alkali-metal salts of strong acids. However, it isnot essential that the salt be neutral in the sense that when dissolvedin water it produces exactly pH 7. A slight departure from completeneutrality does not interfere. Of course, strongly acidic salts are notuseful.

The useful salts apparently are not acting as alkaline catalysts. Thosewhich display some slight alkalinity in water, such as sodium benzoate,do so to a degree of alkalinity insufficient for useful catalysis. Infact, the effectiveness of such salts is not as great as those that aremore nearly completely neutral, such as the alkali metal iodides andthiocyanates. The useful group includes such neutral to slightlyalkaline salts as lithium, sodium and potassium iodide; lithium andsodium perchlorate; the alkali metal salts of such aryl sulfonic acidsas benzene-, tolueneand xylene-sulfonic acids; and the alkali metalthiocyanates and benzoates.

While the salts used in the present invention are hydrotropes,hydropicity is not a critical factor. The relative effectiveness ofdifferent salts is in no sense directly proportional to theirhydrotropic powers. Other factors play an important role. The swellingeffect of the salt on the cellulose is important and well may be themajor factor. Since the exact mechanism is not known the invention isnot intended to be limited to any particular theory.

As noted above, this invention produces an increase in reactivity. itmay be measured, for example, by the increased carbamylethylationobtainable under fixed conditions. Identical reaction conditions which,without salts, yield no more than two percent nitrogen, yield productsof four or more percent nitrogen when the salts are presout duringreaction. Moreover, these new products are substantially water-soluble.In other words, reaction conditions which would normally produce aninsoluble product or a product soluble only in aqueous alkali solution,yield products with a nitrogen content of from three to about sixpercent nitrogen with the salt present.

The present invention also presents marked advantages in increasing theefiiciency of reaction and in shortening the reaction time required toobtain products of less than the maximum nitrogen content. It is thus animportant advantage of the present invention that it is extremelyflexible and better results can be obtained without introducingundesirable characteristics.

The amount of salt which is added can be varied over wide ranges. Thus,for example, sodium iodide may be used in amounts from as little as 20grams, or less per 100 grams of sodium hydroxide solution, up to amountsas high as about 230 grams, which is a saturated solution at C. Ingeneral, the more active salts, of which sodium thiocyanate and sodiumiodide are typical, reach a ver favorable concentration well belowsaturation. Additional amounts increase the reaction efiiciency ratioonly a little. For example, in the case of sodium iodide at 55 C., themaximum efiect is reached at about 100 grams per 100 grams ofNa-OH'solution and adding more does not produce a commensurate increasein effect.

in general then, it is desirable to add as little salt as practicableand still obtain the desired degree of carbamylethylation, in areasonable time with a good carboxyethylation ratio. The minimum usefulamount will be that which will produce swelling of the cellulose inaddition to the degree of swelling produced by the alkali. There is nomaximum since, if so desired, an amount in excess of that which willproduce saturation may be present. Very precise control is notessential. This large range of operative concentrations is an additionaladvantage of the invention.

As noted above, one of the most important features of the presentinvention is the production of the desired degree of carbamylethylationwith a greatly reduced degree of carboxyethylation. This has beenreferred to above as the carboxyethylation ratio. Since it is a relativeterm, the units are not important as long as they are consistent. Onereadily expressed numerical ratio is that of the carboxyethyl content,in milli-equivalent per gram of product, to weight percent nitrogencontent. By the use of the present invention, as noted above, this ratiomay be reduced by 100% or more below those previously possible, making amuch better and efficient usage of the acrylamide or its derivative.This will be illustrated in the following examples wherein all parts andpercentages are by weight, and all temperatures in C., unless otherwisenoted.

Example 1 Five parts of cotton linters are heated with stirring for onehour at C., in a solution consisting of 88 parts of Water, 2 parts ofsodium hydroxide and 10 parts of acrylamide. A trace ofN-phenyl-beta-naphthylamine is also present to prevent polymerization ofthe acrylamide. Neutralization is effected with dilute acetic acid andthe product is thoroughly washed with water and alcohol. Kjeldahlanalysis showns a nitrogen content of only 0.5%. The carboxyethylcontent is 0.05 milliequivalent per gram of product and the carboxyethylratio is about 1:10.

Example 2 To demonstrate the effect of the added salt, the procedure ofExample 1 is repeated exactly with the exception that the parts ofsodium thiocyanate is omitted. The nitrogen content is 1.3% and thecarboxyethyl content is only 0.05 rnilliequivalents per gram, a ratio ofabout 1:10.

The use of the added salt has produced an increase in nitrogen contentof about with a greatly improved carboxyethyl ratio.

Example 3 Twenty-three parts of alpha-cellulose from Wood pulp isallowed to swell for 45 minutes at room temperature in a solution madeup of 295 parts of 2% sodium hydroxide solution, 295 parts of sodiumthiocyanate and a trace of polymerization inhibitor. Acrylamide, 147parts, is then added and the mixture stirred at 7278 C. for 3 hours. Thesolution is then poured into a large excess of methanol, filtered andwashed with more alcohol. The product is almost completely water-solubleboth in basic and acidic solutions, which distinguishes it fromcarboxyethyl cellulose which is soluble only in basic solution. Thenitrogen content is 4.5%; the carboxyethyl content only 0.22milliequivalents per gram, a ratio of about 1:21.

Example 4 Four parts of alpha-cellulose is stirred for about 10 minutesin a solution comprising 1 part of sodium hy droxide, 49 parts of Waterand 50 parts of sodium lithiocyanate. Thirty parts of acrylamidecontaining hydroquinone as polymerization inhibitor is then added andreaction allowed to proceed at 80 C. for 2 hours. The mixture is thenneutralized with an alcoholic solution of acetic acid. The product isthoroughly washed with alcohol, then reslurried in a small quantity ofwater and washed again with alcohol. Analysis shows a nitrogen contentof 3.8%. The carbamylethylated cellulose is partially soluble in Water.

Example 5 Ten parts of cotton linters is allowed to react for 2 hours at80 C., in a solution consisting of 2 parts of sodium hydroxide, 200parts of water, 200 parts of potas sium iodide, 50 parts of acrylamideand a trace of hydroquinone. After workup as described in Example 5, theproduct contains 1.2% nitrogen.

Example 6 Five parts of alpha-cellulose is treated at 80 C., for 2 hourswith a solution consisting of 2 parts of sodium hydroxide, 98 parts ofwater, 35 parts of sodium thiocyanate, 25 parts ofN,N-methylenebisacrylamide and a trace of hydroquinone. After workup asdescribed in Example 5, the product contains 2.9% nitrogen.

Example 7 Five parts of cotton linters are allowed to react in asolution comprising 2 parts of sodium hydroxide, 70 parts of water, 35parts of sodium thiocyanate, 30 parts of N-methylolacrylamide and atrace of hydroquinone. After workup as described in Example 5, theproduct contains 0.6% nitrogen.

Example 8 Example 6 is repeated substituting for theN,N-methylenebisacrylamide an equal weight of N-n-propylacrylamide.Paper made from treated cellulose is stronger than from the untreatedpulp.

As may be noted in the above examples, it is desirable to insure againstpolymerization of the acrylamide, or substituted acrylamide. This may bedone by addition to the treating liquor of a known polymerizationinhibitor such for example as the naphthylamine, hydroquinone used inthe examples and their known equivalents.

The products of this invention have utility in a wide number of fields,for example in producing paper or fabrics of increased strength. Thewater-solub1e products have particular utility as thickners for printingpastes and the like, in coating and sizing compositions, as flocculatingagents and the like.

We claim:

1. In a process of carbamylethylating cellulose by heating it With anaqueous treating liquor comprising water, an acrylamide and a strongalkali, the improvement which comprises; carrying out the reaction atfrom about 45 to about 130 C.; maintaining in said treating liquor, perhundred parts of water, an amount of alkali stoichiometricallyequivalent to from about one-half to about four parts of sodiumhydroxide, an amount of an acryl amide of from about five parts to aboutthe solubility limit at the operating temperature, and an amounteifective to produce swellin of the cellulose of a watersoluble salt ofa strong alkali and a strong acid, said salt having in Water alone asubstantially neutral to slightly alkaline pH and being in said treatingliquor substantially unreactive with the acrylamide; maintaining saidcellulose in intimate reactive contact with a suflicient amount of saidtreating liquor to thoroughly wet the cellulose; and maintaining saidconditions until substitution is completed, whereby the amide-containingcellulose is obtained with a minimized carboxyethyl content.

2. A process according to claim 1, wherein said acrylamide is selectedfrom the group consisting of acrylamide and the N-alkyl substitutedacrylamides, N,N-alkylene bisacrylamides and N-alkylolacrylamideswherein said alkyl, alltylene and alkylol substituents contain from oneto four carbon atoms.

3. A process according to claim 1 in which the cellulose is cotton.

4. A process according to claim 1 in which the cellulose is derived fromWood pulp.

5. A process according to claim 1 in which the cellulose is aregenerated cellulose.

6. A process according to claim 1 in whi h the salt is an alkali metaliodide.

7. A process according to claim 6 in which the reaction is carried outin the temperature range of between about 55 and about C.

8. A process according to claim 1 in which the salt is an alkali metalthiocyanate.

9. A process according to claim 8 in which the reaction is carried outat a temperature in the range between about 55 and about 95 C.

l0. A process according to claim 1 in which the reaction is continueduntil the carbamylethylation has proceeded beyond a nitrogen content ofabout three percent.

11. The process according to claim 10 in which the salt is an alkalimetal iodide.

12. A process according to claim 10 in which the salt is an alkali metalthiocyanate.

13. A process according to claim 10 in which the salt is an alkali metalsalt of a mononuclear aryl sulfonic acid.

14. A process according to claim 10 in which the salt is an alkali metalxylene sulfonate.

References Cited in the tile of this patent UNITED STATES PATENTS Rockat al. Jan. 4, 1944 Gardner Feb. 8, 1949 OTHER REFERENCES

1. IN A PROCESS OF CARBAMYLETHYLATING CELLULOSE BY HEATING IT WITH ANAQUEOUS TREATING LIQUOR COMPRISING WATER, AN ARCYLAMIDE AND A STRONGALKALI, THE IMPROVEMENT WHICH COMPRISES; CARRYING OUT THE REACTION ATFROM ABOUT 45* TO ABOUT 130* C; MAINTAINING IN SAID TREATING LIQUOR, PERHUNDRED PARTS OF WATER, AN AMOUNT OF ALKALI STOICHIOMETRICALLYEQUIVALENT TO FROM ABOUT ONE-HALF TO ABOUT FOUR PARTS OF SODIUMHYDROXIDE, AN AMOUNT OF AN ACRYLAMIDE OF FROM ABOUT FIVE PARTS TO ABOUTTHE SOLUBILITY LIMIT AT THE OPERATING TEMPERATURE, AND AN AMOUNTEFFECTIVE TO PRODUCE SWELLING OF THE CELLULOSE OF A WATERSOLUBLE SALT OFA STRONG ALKALI AND A STRONG ACID, SAID SALT HAVING IN WATER ALONE ASUBSTANTIALLY NEUTRAL TO SLIGHTLY ALKALINE PH AND BEING IN SAID TREATINGLIQUOR SUBSTANTIALLY UNREACTIVE WITH THE ARCYLAMIDE; MAINTAINING SAIDCELLULOSE IN INTIMATE REACTIVE CONTACT WITH A SUFFICIENT AMOUNT OF SAIDTREATING LIQUOR TO THROUGHLY WET THE CELLULOSE; AND MAINTAINING SAIDCONDITIONS UNTIL SUBSTITUTION IS COMPLETED, WHEREBY THE AMIDE-CONTAININGCELLULOSE IS OBTAINED WITH A MINIMIZED CARBOXYETHYL CONTENT.